Initially derived from the phrase "bull shit airy". This phrase was used by herders of flying cows to describe cows that shit in mid flight, this later came to mean any bullshit that was higher in altitude then normal, and later to its current meaning of any bullshit that is bullshitier then normal.

person 1: "fred said some bullshit last week"
person 2: "actually that bullshit exceeded the limit for normal bullshit promoting it to bullshitery"